sql命令从数量中减去一项

我是sql的新手，真的不知道怎么做。我有两张桌子。卡车

 select * from trucks;
+----+---------+----------+--------+--------------+
| id | size    | quantity | status | customers_id |
+----+---------+----------+--------+--------------+
| 12 | 10_feet | 3        | active |            0 |
| 13 | 10_feet | 3        | active |            0 |
| 14 | 10_feet | 2        | active |            0 |
| 15 | 14_feet | 5        | active |            0 |
| 16 | 14_feet | 2        | active |            0 |
| 17 | 14_feet | 2        | active |            0 |
| 18 | 17_feet | 2        | active |            0 |
| 19 | 17_feet | 2        | active |            0 |
| 20 | 24_feet | 3        | active |            0 |
| 21 | 10_feet | 1        | active |            0 |
| 22 | 24_feet | 1        | active |            0 |
+----+---------+----------+--------+--------------+
and truck_customers
select * from truck_customers;
+----+--------+----------+--------+------------------+---------+------------+
| id | first  | last     | dl     | email            | size    | date_in    |
+----+--------+----------+--------+------------------+---------+------------+
|  3 | Poul   | Jons     | A13324 | poul@gmail.com   | 10_feet | 2013-11-30 |
|  4 | Poul   | Watson   | A23439 | watson@gmail.com | 10_feet | 2013-11-30 |
|  5 | Alex   | Snders   | A22    | Alex@gmail.com   | 17_feet | 2013-11-30 |
|  6 | santes | Garsia   | A18337 | Santes@gmail.com | 10_feet | 2013-11-30 |
|  7 | James  | Bond     | JB111  | Bond@gmail.com   | 10_feet | 2013-11-30 |
|  8 | John   | Travolta | G123   | Gohn@gmail.com   | 14_feet | 2013-11-30 |
+----+--------+----------+--------+------------------+---------+------------+

在truck_customers中输入信息时，应自动减去根据尺寸从卡车表中选择一项。

使用

$sql = " UPDATE trucks SET quantity = --  WHERE size = '$size'";

但它不起作用。

如果我的代码更容易理解，我可以把它发给你。